As semiconductor device manufacturers continue to produce smaller devices, the requirements for photomasks used in the fabrication of these devices continue to tighten. Photomasks, also known as reticles or masks, typically consist of substrates (e.g., high-purity quartz or glass) that have a non-transmissive layer (e.g., chrome) formed on the substrate. The non-transmissive layer includes a pattern representing a circuit image that may be transferred onto a semiconductor wafer in a lithography system. As feature sizes of semiconductor devices decrease, the corresponding circuit images on the photomask also become smaller and more complex. Consequently, the quality of the mask has become one of the most crucial elements in establishing a robust and reliable semiconductor fabrication process.
The quality of a photomask is typically controlled by a specification that provides the requirements that a photomask must meet in order to be used to fabricate semiconductor devices on a wafer. For example, a specification may include the requirements for pattern position accuracy, feature size control, defect density and phase shift tolerance for a specific manufacturing process.
The pattern position accuracy, feature size control and phase shift tolerance on an individual photomask may be measured by a metrology tool. Typically, a metrology tool should have no more than ten percent error relative to the specification in order to provide quality assurance. A metrology tool, therefore, must be able to precisely and accurately measure a feature on a photomask in order to meet the requirements in the specification.
Standards used to calibrate different metrology tools have been created in order to ensure the precision and accuracy of the measurements obtained by the metrology tools. Currently separate standards exist for metrology tools, such as a profilometer and an atomic force microscope (AFM), that measure the vertical profile of a feature on a substrate or wafer. For example, a chrome pattern on an etched quartz substrate may be used for the profilometer and a platinum coated SiO2 pattern on a silicon substrate may be used for the AFM. These standards may be used to calibrate the tools to a specification for a specific manufacturing process. Unlike profilometers and AFMs, no standard currently exists for metrology tools that measure a phase shift created by a photomask. Currently, a manufacturer may use control vehicles having arbitrary phase shifts to calibrate their phase metrology tools without knowing if the original phase of the control vehicle was correct. This approach has two problems. First, the accuracy of the calibration is about two degrees because the lithography is not very sensitive to phase error. Second, the results may be effected by spherical lens error in the lithography tool.